The present invention relates to an improved apparatus and method for compensating for distortion in an ink jet printing apparatus.
In general, ink jet printing apparatus produce a series of ink droplets which are successively charged, deflected electrostatically, and conveyed with a trajectory towards a recording medium to perform printing by a series of dots thereon in response to print information. During the time when the respective ink droplets are conveyed, the respective ink droplets effect the flow of air behind them. When a subsequent ink droplet enters into the flow of air, the aerodynamic resistance acting on the subsequent ink droplet becomes smaller. In consequence, the preceding and subsdquent ink droplets may move closer to each other or may combine into one droplet while being conveyed toward the recording, causing distortion of the printed image or character.
Moreover, the respective ink droplets for printing are typically charged by an amount corresponding to the magnitude of the print or charging signal, and a Coulomb's force (electrostatic repulsive force) thus acts between the respective charged ink droplets. This electrostatic repulsive force may further disturb the distance between the respective ink droplets, causing a distortion in the information printed on the record medium. Further, under the influence of the preceding charged ink droplets, the amount of charge applied to the ink droplet just about to be charged may be decreased, thereby causing a similar distortion. For the purpose of solving the afore-mentioned defects, a method as shown in the U.S. Pat. No. 3,946,399 has already been proposed, for example. The invention disclosed in the U.S. Pat. No. 3,946,399 relates to a method of compensating for the charging amount, wherein the pattern to be printed is detected in advance and signals are developed to compensate for the expected distortion of the deflection of the ink droplets due to the mutual influence of the Coulomb's force working between the respective charged ink droplets and the aerodynamic resistance variation, and the charging amount is compensated for in response to these signals.
However, this method is not efficient in the case where many deflection steps, 32 steps for example, are possible for the ink droplets. The reason for that is as follows. As the number of deflection steps increase, the respective ink droplets are positioned more closely together so that the amount of print distortion may increase. In addition, due to the different flight time of each ink droplet which is dependent on the amount of deflection, the amount of distortion differs depending on the amount of deflection. In order to properly compensate for the print distortion, therefore, it is necessary to perform appropriate compensation for each deflection step. The afore-mentioned deflects often present rather difficult problems in the ink jet printing technology.